The present invention relates to an ultrasonic irradiation system such as an ultrasonic heater, an ultrasonic chemical reaction accelerator or a sonicator and, more particularly, to an ultrasound therapy system suitable for therapy of malignant tumors.
In each application field of the ultrasonic irradiation including a medical treatment, an ultrasound focal zone of single spot focus may be too small, as compared with a target zone to be irradiated. As an ultrasonic irradiation system suited for this application, there has been known in the art a system for forming an annular focal zone by using such an acoustic lens as is disclosed in "Ultrasound in Med. & Biol." vol. 8, No. 2 (issued in 1982), pp. 177 to 184. However, this system is encountered with an inconvenience that the position, size and shape of irradiation cannot be controlled in accordance with an object to be irradiated with ultrasonic waves because they are fixed. Moreover, the system has a tendency that the ultrasonic waves spreading again from an annular focal zone A will reform a long column-shaped focal zone B on the center axis of the annulus, as shown in section in FIG. 1. This tendency is strengthened to raise a serious problem especially in case the diameter of the annulus is smaller than that of the ultrasonic probe. In the field of ultrasound therapy, for example, the secondary focal zone other than the target zone to be irradiated may adversely affect the normal internal tissues of a patient or may cause pain in him.